1. Field of the Invention
This invention relates to a method of simultaneously generating power and refrigeration from low-grade thermal energy, using hydrogen and materials which reversibly form a hydride therewith at low temperature and pressure, and which reversibly dehydride to release large quantities of hydrogen at a relatively higher temperature and pressure, which hydrogen is then used to directly produce power and yield refrigeration in a thermodynamic cycle which includes a gas expansion device.
2. Brief Description of the Prior Art
Various types of power cycles using a number of working fluids have been heretofore proposed. In one long-known and widely used system, heat energy in a working fluid is used in a Rankine cycle to drive a turbine to produce power. The cycle is completed through the sequence of condensing the working fluid, compressing the liquid, and evaporating the liquid to a high pressure vapor state. Various fluids which may be easily condensed near ambient temperature, such as water, iso-butane and chlorinated hydrocarbons, can be used as the working fluid.
In another long-known and widely used power system, gases are used as the working fluid in a Brayton cycle to drive a turbine. In a closed system, the cycle is completed by effecting recompression of the gas through the use of a compressor and heat enchangers. This type of power system differs basically from the Rankine cycle system by the nature of the working fluid and the manner by which it is recompressed. This type of power system is well known in the field of thermodynamics and is used in many high-temperature energy conversions, such as in jet engines where an open system is used and in high-temperature gas-cooled nuclear power plants.
Various types of refrigeration cycles using a number of working fluids have been heretofore proposed. In one long-known and widely used system, refrigeration is secured by vapor-compression of the working fluid with subsequent cooling to the liquid state, expansion of the liquid through an expansion valve with refrigeration being produced in that portion of the cycle where evaporation occurs. Working fluids commonly used in this vapor-compression cycle are chlorinated hydrocarbons.
In the air-standard refrigeration cycle, the phases of the cycle are essentially the reverse of those encountered in the Brayton cycle. Air is initially compressed and is then cooled before being expanded to further lower its temperature. This type of cycle is used in low-pressure air liquefaction plants, and in other liquefaction devices, such as the Collins helium liquifier.
In the absorption refrigeration cycle, absorption of the working fluid occurs at a low pressure and temperature with release at a relatively higher pressure and temperature. Subsequent cooling to the liquid state and expansion through an expansion valve produce refrigeration. Ammonia-water and lithium bromide-water are common working fluid-absorber combinations.
Various types of working fluids for use in power and refrigeration cycles have been heretofore proposed. In the Stirling cycle, hydrogen has been proposed as a working fluid which is directed in sequence to a compressor, heat exchanger and expansion device in a manner analogous to the Brayton cycle.
In U.S. Pat. No. 3,370,420 to Johnson, working fluids which thermally dissociate upon heating, and reversibly recombine in the gas phase upon cooling, are proposed for several closed cycle thermodynamic processes (Brayton, Stirling and Rankine). The patentee asserts that any non-dissociating gas, when used as a working fluid, severely restricts the efficiency of a power cycle.
A good discussion of the various advantages and disadvantages which have characterized many types of working fluids previously used in power cycles is contained in U.S. Pat. No. 3,152,357. All the described fluids are types which undergo a change of state from liquid to vapor in the course of the power cycle. An ideal working fluid is described as one having good thermal stability, high cycle efficiency, lower corrosiveness, favorable critical properties and low toxicity.
A proposal for developing power from a system comprising a multiplicity of power fluids is described in U.S. Pat. No. 3,395,103 to Anderson. Direct use of a relatively low temperature energy source, such as geothermal water, to heat and vaporize a working fluid for expansion through a turbine for the development of power has been hindered by the large cost of the heat exchanger required in order to effect heat transfer. The patentee proposes to eliminate a portion of this cost by adding a second power plant of the Rankine cycle type and using the residual heat in the hot primary fluid to heat and vaporize a different working fluid having a lower boiling point than that used in the first cycle.
Recently, the properties and useful application of various metal hydrides have been considered in power and refrigeration cycle systems. A considerable amount of work in this respect has been done under the auspices of the U.S. Atomic Energy Commission. The ability of hydrides to chemically store hydrogen at a relatively low temperature and pressure in a concentrated form, and then to release the hydrogen at an elevated temperature and pressure has been recognized, and a number of hydride-forming materials have been identified.
In U.S. Pat. No. 3,508,414 to Wiswall and Reilly, a method of storing hydrogen is described in which gaseous hydrogen is absorbed by titanium-iron alloys. When such a hydride containing 2 weight percent hydrogen is maintained at a temperature of 25.degree.C, hydrogen is released at a constant rate until less than 1.0 weight percent of the hydrogen remains in hydride form. In U.S. Pat. No. 3,315,479 to Wiswall and Reilly, a method of storing hydrogen by formation of nickel-magnesium hydride is discussed. Similar formation of copper-magnesium hydrides is discussed in U.S. Pat. No. 3,375,676, issued to the same patentees. In U.S. Pat. No. 3,516,263, Wiswall and Reilly further discuss the formation of titanium-iron hydrides, and point out that a particular type of pressure vessel may be used to contain the hydride, and to heat the hydride to develop hydrogen pressures exceeding 10,000 psi.
By alternating the formation and decomposition of the metal hydride, workers at the Brookhaven National Laboratory have proposed, in Report No. 15844, April, 1971, to use the alternate decomposition and regeneration of the hydride as a gas circulation pump. Such systems have also been proposed for refrigeration.
More recently, in U.S. Pat. No. 3,504,494, a closed cycle method for intermittently producing high energy steam has been described. In the system here reported, a power cycle, followed by a recharging cycle, is utilized. In the power cycle, a first hydride bed is heated to desorb hydrogen gas therefrom. The gas flows to a second hydride bed where the hydrogen can be absorbed at a lower temperature than the temperature of desorption from the first bed. Absorption of the hydrogen by the second bed releases the heat of absorption which is used to convert water to steam. The steam is used for power production, and the residual heat remaining in the steam after such power production is used for heating the first hydride bed and enhancing the desorption of hydrogen therefrom. After complete desorption of the hydrogen from the first bed and condensation of the residual steam, the recharging cycle is started. In the recharging cycle, the second hydride bed is heated by a heat source which can be a low energy isotope source, a chemical heater, an electrical heater or other suitable source of thermal energy. The second bed is thus caused to dehydride, and the first bed is cooled so that it can absorb the hydrogen desorbed from the second bed preparatory to recommencing the power cycle after recharging.
An integral and essential part of this review of prior art is the lack of any finding substantiating the fact that both power and refrigeration can be practically generated from low temperature energy sources except in a conventional manner as referred to above in the examples of the use of iso-butane and chlorinated hydrocarbons. Although the prior art recognizes the possible use of hydrides in a gas circulation pump, the art does not contemplate the economic or practical or technical feasibility of large-scale commercial power generation and refrigeration, and the need for energy conservation achieved through extraction of energy from low-temperature energy sources as defined hereinbelow as the subject of this invention. it remained for the present applicants to recognize these circumstances and apply properties of hydrides and hydrogen to the preparation of a system capable of extracting power and refrigeration from low-temperature energy sources.